To date, in electric power steering apparatuses in each of which an electric motor produces assist torque for assisting steering torque produced by a vehicle driver, there exists an electric power steering apparatus in which there is provided an electric motor having two sets of armature windings that are formed substantially in the same manner and in which an inverter circuit that can separately drive the two sets of armature windings is provided in a control unit. At a normal time, the conventional electric power steering apparatus configured in such a manner as described above makes two sets of inverter circuits collaborate with each other so as to control the electric motor; however, in the case where an abnormality occurs in one of the two sets of inverter circuits, only the other normal inverter circuit continues driving of the electric motor.
To date, there has been disclosed an electric power steering apparatus in which the control-unit constituent elements, other than the inverter circuit, are made to be a double system so that the redundancy is raised and hence the countermeasure for a failure is further enriched (for example, refer to Patent Document 1). In the conventional electric power steering apparatus disclosed in Patent Document 1, not only two sets of armature windings of an electric motor are provided but also two sets of control units, each of which has a +B power source to be connected with the positive-polarity electrode of a vehicle battery, an input circuit, a CPU, an output circuit, and the like, are provided; in the case where an abnormality occurs in one of the two sets of control units, the other normal one thereof can continue driving of the electric motor.